The present invention relates to a radiation image conversion panel employing stimulable phosphors, and an image formation method using the panel. Specifically the present invention relates to a radiation image conversion panel which results in excellent balance between the emission luminance and the sharpness of said stimulable phosphors.
Radiation images such as X-ray images are widely employed for medical diagnoses. Utilized as a method for obtaining X-ray images is so-called radiography in which X-rays, which have passed through an object, are subjected to irradiation onto a phosphor layer (being a fluorescent screen) to result in visible light, which is irradiated onto a silver salt bearing film, in the same manner as conventional photography, and the resulting film is subjected to photographic processing.
In recent years, however, a method has been invented in which images are formed directly from a phosphor layer instead of an image formation method employing a silver salts photographic film.
This method comprises the steps of (1) making absorbing the radiation energy which passes through the object to the phosphor; and (2) stimulating the phosphor with light or heat so that radiation energy stored in said stimulable phosphor layer is released as stimulated luminescence. (3) forming images after detecting the released energy.
Said method is described, for example, in U.S. Pat. No. 3,859,527 and Japanese Patent Publication Open to Public Inspection No. 55-12144. A radiation image conversion panel comprised of stimulable phosphors is disclosed in them.
This method uses a radiation image conversion panel and the stimulable phosphor layer of said radiation image conversion panel is subjected to radiation exposure which passes through the object being diagnosed so that radiation energy is stored corresponding to the radiation transmittance of each portion of said object. Subsequently, the resulting stimulable phosphor layer is sequentially subjected to stimulation employing electromagnetic waves (stimulating light), such as visible light and infrared rays, so that radiation energy stored in said stimulable phosphor layer is released as stimulated luminescence. Signals of the intensity variation of said stimulated luminescence are subjected, for example, to photoelectric conversion to obtain electrical signals. The resulting electrical signals are employed to reproduce visible images on recording materials such as light-sensitive films or display devices such as a CRT.
The above-mentioned image reproducing method has an advantage of using much less amount of radiation exposure compared with the conventional radiography using a combination of an intensifying screen and a conventional radiographic film. It is possible to obtain radiation images with ample information.
The stimulable phosphors employed in said radiation image conversion panel are those which result in stimulated luminescence after having been subjected to irradiation of stimulating light after said radiation. In practice, phosphors are commonly employed which result in stimulated luminescence in the wavelength range of 300 to 500 nm utilizing stimulating light in the wavelength region of 400 to 900 nm.
The radiation image conversion panel, employing said stimulable phosphors, stores radiation image information and releases stored energy through stimulating light scanning. Therefore, after scanning, it is possible to repeatedly store radiation images so as to be used repeatedly. Further, contrary to the fact that in the conventional radiography, radiographic film is consumed for every exposure, said radiation image conversion method is more advantageous from the viewpoint of resource conservation as well as economic efficiency, because it is possible to repeatedly utilize said radiation image conversion panel.
Relative advantages of radiation image conversion systems employing a radiation image conversion panel vary to large extent depending on the luminance (occasionally called sensitivity) of stimulated luminescence, as well as the image quality represented by the resultant graininess and sharpness, and these characteristics vary widely depending on characteristics of used stimulable phosphors and the configuration of the stimulable phosphor layer. In more detail, the luminescence intensity of the radiation image conversion panel, the sharpness of images, and the granularity vary depending on the size of phosphor particles, the dispersibility of said phosphors, the uniformity of phosphors, and the phosphor filling ratio. Among those, the phosphor filling ratio results in pronounced effects.
As a means to enhance said filling ratio, Japanese Patent Publication Open to Public Inspection No. 3-21893 discloses a radiation image conversion panel having a stimulable phosphor filling ratio of at least 70 percent, while employing a resin having a glass transition temperature (hereinafter occasionally referred to as Tg) of 30 to 150xc2x0 C., and as an achieving means, discloses the compression of a phosphor layer (hereinafter referred simply to as a coating). The radiation image conversion panel, when employed, is slid with films as well as rollers. As a result, it is assumed that the Tg of employed binder resins is preferably at least 30xc2x0 C. However, when resins having a relatively high Tg are employed as a binder resin, it becomes difficult to increase said filling ratio due to the fact that the resulting coating is not easily deformed. Further, when the finished coating is compressed, phosphors are subjected to loading due to poor softening properties of said resin, whereby light emission is degraded due to the destruction of the crystal structure of the luminous body. Further, in order to soften said resins, it is necessary to increase the compression temperature. As a result, problems have occurred in which manufacturability is degraded.
Further, Japanese Patent Publication Open to Public Inspection No. 4-44719 discloses a method to enhance the filling ratio of a phosphor layer utilizing a compression treatment. However, said patent publication does not describe any material in regard to the roller employed for said compression treatment. When simply passed between heating rollers, it was found that compression was insufficient and the phosphor was damaged. Further, said patent publication presents no suggestion in regard to the shape of the roller used. On the other hand, it was found that in the compression treatment employing a linear-shaped calender roller, said calender roller tended to warp resulting in unevenness of the compression ratio of the phosphor layer. Said unevenness induces an unevenness of the thickness of the phosphor layer, which results in sharpness fluctuation as well as granulated unevenness of the radiation image conversion panel. As a result, prompt improvement has been demanded.
In the aforesaid patent, the condition is that the temperature during heat compression is more than or equal to the Tg of a resin. For example, the condition such as 80xc2x0 C., 100xc2x0 C., or the like, is described. The temperature more than or equal to the Tg of said resin, as described herein, is 69xc2x0 C., which is the Tg of polyethylene terephthalate film employed widely as a support, or more than that. When a compression treatment is carried out at a temperature more than or equal to the Tg of such a support, problems occur in which said support is deformed and finally the stimulable phosphor plate is also deformed. Particularly, the deformed plate results in unevenness of luminance and sharpness during reading of images, and further results in critical problems with diagnosis. As a result, rapid improvement of these problems has been demanded.
From the view of the foregoing, the present invention has been achieved. An object of the present invention is to provide a radiation image conversion panel which exhibit an excellent balance of luminance and sharpness, and in addition, minimal sharpness fluctuation, a production method thereof, and a radiation image capturing method using the same.
Said object of the present invention was achieved employing the embodiments described below.
(1) A method for preparing a radiation image conversion panel, which comprises the steps of:
(a) applying onto a support a stimulable phosphor coating composition comprising a stimulable phosphor and a polymer resin to form a stimulable phosphor layer;
(b) drying the stimulable phosphor layer; and
(c) subjecting the stimulable phosphor layer on the support to a compression treatment employing a calender roller which comes into contact with the stimulable phosphor layer to form the radiation image conversion panel, wherein the calender roller comprises a resin and the surface of the calender roller has a Shore D hardness of D80 to D97xc2x0.
(2) A method for preparing a radiation image conversion panel, which comprises the steps of:
(a) applying onto a support a stimulable phosphor coating composition comprising a stimulable phosphor and a polymer resin to form a stimulable phosphor layer;
(b) drying the stimulable phosphor layer; and
(c) subjecting the stimulable phosphor layer on the support to a compression treatment employing a calender roller which comes into contact with the stimulable phosphor layer to form the radiation image conversion panel, wherein the calender roller has a crown value of 10 to 1,000 xcexcm.
(3) A method for preparing a radiation image conversion panel, which comprises the steps of:
(a) applying onto a support a stimulable phosphor coating composition comprising a stimulable phosphor and a polymer resin to form a stimulable phosphor layer, wherein the polymer resin comprises a polymer having a glass transition point of not more than 5xc2x0 C. and not less than xe2x88x9230xc2x0 C. and the polymer accounts for at least 50 weight % of the polymer resin in the stimulable phosphor layer;
(b) drying the stimulable phosphor layer; and
(c) subjecting the stimulable phosphor layer on the support to a compression treatment employing a calender roller which comes into contact with the stimulable phosphor layer to form the radiation image conversion panel, wherein the temperature of the calender roller is not less than the glass transition temperature of the polymer resin and not more than a glass transition temperature of the support.
(4) The method for preparing a radiation image conversion panel of item 1, wherein the polymer resin in the step (a) comprises a polymer having a glass transition point of not more than 5xc2x0 C. and not less than xe2x88x9230xc2x0 C. and the polymer accounts for at least 50 weight % of the polymer resin in the stimulable phosphor layer; and the temperature of the calender roller in the step (c) is not less than the glass transition point of the polymer resin and not more than a glass transition point of the support.
(5) The method for preparing a radiation image conversion panel of item 2, wherein the polymer resin in the step (a) comprises a polymer having a glass transition point of not more than 5xc2x0 C. and not less than xe2x88x9230xc2x0 C. and the polymer accounts for at least 50 weight % of the polymer resin in the stimulable phosphor layer; and the temperature of the calender roller in the step (c) is not less than the glass transition point of the polymer resin and not more than a glass transition point of the support.
(6) The method for preparing a radiation image conversion panel of item 1, wherein the compression treatment in the step (c) is carried out at a pressure of 500 to 5,000 N/cm and at a temperature of 50 to 150xc2x0 C.
(7) The method for preparing a radiation image conversion panel of item 2, wherein the compression treatment in the step (c) is carried out at a pressure of 500 to 5,000 N/cm and at a temperature of 50 to 150xc2x0 C.
(8) The method for preparing a radiation image conversion panel of item 3, wherein the compression treatment in the step (c) is carried out at a pressure of 500 to 5,000 N/cm.
(9) The method for preparing a radiation image conversion panel of item 1, wherein the calender roller in the step (c) has a center-line mean surface roughness Ra of 0.05 to 3 xcexcm.
(10) The method for preparing a radiation image conversion panel of item 2, wherein the calender roller in the step (c) has a center-line mean surface roughness Ra of 0.05 to 3 xcexcm.
(11) The method for preparing a radiation image conversion panel of item 3, wherein the calender roller in the step (c) has a center-line mean surface roughness Ra of 0.05 to 3 xcexcm.
(12) The radiation image conversion panel prepared according to the method of item 1.
(13) The radiation image conversion panel prepared according to the method of item 2.
(14) The radiation image conversion panel prepared according to the method of item 3.
(15) The radiation image conversion panel of item 12, wherein the stimulable phosphor incorporated in the stimulable phosphor layer is an Eu added BaFI compound.
(16) The radiation image conversion panel of item 13, wherein the stimulable phosphor incorporated in the stimulable phosphor layer is an Eu added BaFI compound.
(17) The radiation image conversion panel of item 14, wherein the stimulable phosphor incorporated in the stimulable phosphor layer is an Eu added BaFI compound.
(18) A method for capturing a radiation image, which comprises the steps of:
(a) irradiating the radiation image conversion panel of item 12 from the support side of the radiation image conversion panel with X-ray which passes through an object being diagnosed so that to store a radiation energy;
(b) stimulating the stimulable layer with an electromagnetic wave to produce stimulated luminescence; and
(c) reading the stimulated luminescence from the stimulable phosphor layer side.
(19) A method for capturing a radiation image, which comprises the steps of:
(a) irradiating the radiation image conversion panel of item 13 from the support side of the radiation image conversion panel with X-ray which passes through an object being diagnosed so that to store a radiation energy;
(b) stimulating the stimulable layer with an electromagnetic wave to produce stimulated luminescence; and
(c) reading the stimulated luminescence from the stimulable phosphor layer side.
(20) A method for capturing a radiation image, which comprises the steps of:
(a) irradiating the radiation image conversion panel of item 14 from the support side of the radiation image conversion panel with X-ray which passes through an object being diagnosed so that to store a radiation energy;
(b) stimulating the stimulable layer with an electromagnetic wave to produce stimulated luminescence; and
(c) reading the stimulated luminescence from the stimulable phosphor layer side.